Continuous process of synthesizing amino-iminomethane-sulfinic acid



3,355,486 CONTINUOUS PROCESS OF SYNTHESIZING AMINO-IMINOMETHANE-SULFINICS. BERKOWITZ ET AL Nov. 28, 1967 ACID Filed Jan. 8, 1965 THIOUREASOLUTION FEED HYDROGEN PEROXIDE FEED THERMO CO U OVERFLOW TUBE INVENTORSSIDNEY BERKOWITZ MORTON MEADOW BYDONALD S. BUNIN I Z ATTORNEY F I LTRATEUnited States Patent 3,355,486 CONTINUOUS PROCESS OF SYNTHESIZINGAMINO-IMINOMETHANE-SULFHNIC ACID Sidney Berkowitz, Highland Park, MortonMeadow, Trenton, and Donald S. Bunin, Metuchen, N.J., assignors to FMCCorporation, New York, N.Y., a corporation of Delaware Filed Jan. 8,1965, Ser. No. 424,325 Claims. (Cl. 260-513.7)

This invention relates to the synthesis of aminoiminomethane-sulfinicacid, also referred to as thiourea dioxide, which will be referred toherein, for the sake of brevity, as TDO. More particularly, thisinvention relates to a continuous process of synthesizing TDO.

In this specification all percentages are given on a weight basis.

TDO is useful as a reducing agent; it reduces vat dyes, ketones toalcohols and hydrocarbons and conjugated unsaturated acids to thecorresponding saturated acids. It is an excellent antioxidant in thestabilization of perchloroethylene. It is useful in organic synthesis inthe preparation of pharmaceuticals and other chemicals.

The reaction of hydrogen peroxide with thiourea or equilibrium mixturesof ammonium thiocyanate and thiourea to produce TDO is disclosed, forexample, in United States Patents 2,150,921 granted Mar. 21, 1939;2,347,446 granted Apr. 25, 1944; and 2,783,272 granted Feb. 26, 1957.The reaction is an exothermic reaction with liberation of largequantities of heat, about 103K cal. per mol; it proceeds instantaneouslywith the formation of byproducts Which reduce the yield and contaminatethe product. This may explain why procedures known prior to thisinvention employed batch techniques with slow addition of the reactantsto the reaction mixture, in an efiort to minimize the formation ofundesired by-products. For example, in the process disclosed in UnitedStates Patent 2,783,272, 30 parts of a 50% hydrogen peroxide solution isadded at a rate of 1.2 parts per minute to an agitated solution of 20parts of thiourea in 300 parts of Water maintained at 2 C. to 5 C., toproduce a slurry of TDO crystals, and to this slurry was added 120 partsof solid thiourea and 222 parts of 50% hydrogen peroxide solutionseparately. The hydrogen peroxide was added at a rate of about 1.2 partsper minute. The thiourea was added in small increments over the sametime period as required for the addition of the hydrogen peroxide at arate to maintain the slurry at a pH of 3 to 5 and to maintain aconcentration of unreacted thiourea in the liquid phase of the slurrywithin the range of 0.5% to 1.0% during substantially the entirereaction period. Thus the residence time of the reactants and reactionmixture within the reactor in this process was about 6.5 hours; the timerequired for the formation of the initial slurry of TDO was 25 minutes;the time required for the separate addition of the 222 parts of hydrogenperoxide solution and the 120 parts of thiourea to this slurry was 185minutes each, making a total of 395 minutes.

The relatively long residence times for the reactants and reactionmixtures in contact with each other involved in batch procedures,results in the formation of undesired by-products which contaminate theTDO and reduce the yield of TDO and this notwithstanding the slowaddition of the reactants to the reaction mixture. Such long residencetimes, for example, permit hydrogen peroxide to react with the TDO inthe reaction mixture to produce undesired oxidation products.Furthermore, for production on a commercial scale such batch proceduresrequire large and costly equipment for accommodating large volumes ofsolution and occupying considerable floor area.

It is a principal object of the present invention to provide acontinuous process of producing TDO, which results in high yields ofgood purity product and can be carried out in relatively simple, compactequipment.

Other objects and advantages of the present invention will be apparentfrom the following detailed description thereof.

In accordance with this invention, a stream containing thiourea, eitheraqueous thiourea or an equilibrium mixture of thiourea and ammoniumthiocyanate, the stream having a concentration of from about 0.7% toabout 10% of the thiourea, and a stream of aqueous hydrogen peroxidesolution containing hydrogen peroxide in amount to produce uponadmixture with the aqueous thiourea containing stream, a mixturecontaining from about 1.5% to 12% hydrogen peroxide, preferably about 6%hydrogen peroxide, are supplied continuously to a reactor, where thereactants are intimately mixed and reacted at a temperature of fromabout 5 C. to about 80 0, preferably about 5 C. to about 15 C. and at apH of from about 3 to 7. At these temperatures and pH conditions thereaction proceeds readily and rapidly and without excessive formation ofby-products. The reaction mixture is withdrawn continuously andimmediately cooled to a temperature of about 0 C. The rates of flow ofthe reactants into the reactor and that of the reaction mixturewithdrawn therefrom are controlled to provide a residence time withinthe reactor of from about 0.1 second to about 10 minutes, desirably notmore than about 7 minutes. The preferred residence time is about oneminute.

Cooling of the reaction product immediately after leaving the reactorquenches the reaction mixture which is at a higher temperature than thereactor and thus tends to minimize formation of by-products. Thereaction mix ture can be cooled to about 0 C. to reduce the solubilityof the product to a minimum before recovering the TDO crystals from thereaction slurry. Such cooling can be effected when the reaction mixtureis quenched or the reaction mixture can first be cooled by cooling waterto a temperature of about 20 C. and thereafter with ice water to atemperature of about 0 C.

The desired temperature of from about 5 to about 80 C. can be maintainedin the reactor by controlling the rate of feed of the two reactantstreams and, if necessary, by cooling the reactor.

The hydrogen peroxide and thiourea are reacted in approximatelystoichiornetric proportions, i.e., two mols of hydrogen peroxide per molof thiourea. Desirably a small excess, say about 5% of thiourea over andabove the stoichiometric amount is used. Thiourea of commerce or thereaction mixture produced by heating ammonium thiocyanate to produce anequilibrium mixture containing ammonium thiocyanate and 25% thiourea canbe used as the thiourea reactantv In either case whether thiourea, assuch, or the equilibrium mixture of ammonium F thiocyanate and thioureais employed, the thiourea reactant is dissolved in water to produce theaqueous solution containing from about 0.7% to about 10% of thiourea,which solution is fed continuously into the reactor.

Hydrogen peroxide of commerce, in any available concentration, can beused. Desirably 50% hydrogen peroxide concentration in water is used asthe feed stream. The flow rate of the hydrogen peroxide stream iscontrolled to produce a reaction mixture in the reactor containing fromabout 1.5% to about 12% hydrogen peroxide, preferably about 6% hydrogenperoxide.

The pH within the range of about 3 to 7 is critical. Operating at a pHabove 7, i.e., under basic conditions, tends to favor reaction of TDOwith itself or with hydrogen peroxide, both of which reactions arewasteful of the desired product. If acid is added to the reactionmixture, peracids form which react with the TDO, reducing the yield ofthe desired product. For example, peracetic acid is formed when aceticacid is added to the reaction mixture and peroxymonosulfuric acid whensulfuric acid is added to the reaction mixture, both of which peracidsreact with The following examples are given to illustrate the preferredpractice of the process of this invention, without, however, limitingthe invention to these illustrative examples.

the TDO to reduce the yield of desired reaction product. Example I Byfeeding continuously streams of hydrogen peroxide and aqueous solutionsof thiourea or thiourea and am- Thls exehhple was eahned e m theeehlpmeht of the monium thiocyanate, at the controlled rates indicated,type shown In the erawlhg helhg eh aluminum Plpe Cress gives pHconditions of from 5 to 6 in the reaction mixs as shown eeeh leg orwhleh had an of tum flowing through the reacton 1 men. 338 ml. ofhydrogen peroxide of 50% concentra- The reactants are mixed intimatelyand thoroughly tionand 2420 ml. of an aqueous solution of thiourea coninthe reactor. Use of a coil or pipe causes the reactants te1h1hg 1e%thlehrea were Pumped eehhhhehsly Into the to mix intimately, thuspromoting the reaction without obreepeehlie legs 14 and 11 of e cross ata rate of mL/ jectionable formation of byproducts. min. thiourea and 5.7ml./m1n. hydrogen peroxide for In reactors not of tha Pipe type, inwhich the cross one hour. The reaction mixture was wlthdrawnfrom thesectional area of the reactor is relatively large as comleg 19 of theCress e e of mlgper hhhute h pared with the inside diameter of the pipeor pipes of pipe passed through the circulating water bath 1n tank 22mamreactors, the reactor is provided with a suitable mixer or tamed htemperature of The resldehee me In agitator to obtain intimate mixing ofthe reactants and the reaeheh ereee about 30 seconds The tempuerareaction mixture during their flow through the reactor. 20 thre f thereaeheh hhxthre m cross was h The reactor as Well as the remainingequipment dc, to 10 C. lfrom the cooling co1l21 the reaction mixtureSammy is of amminum bgcause aluminum has good was passed into the heatexchanger where it was cooled thermal conductivity and is compatible.Vllh the react- Wlth lee Water to a temperature of about on The TDOants and the reaction mixture in that aluminum will not crystals eremoved by filtefmgfihe resultant react with them or cause them todecompose. Instead 25 The weight of the Product ehtemee (318 grams)based of aluminum, stainless steel or other suitable construcen h Weightof e hydrogen Peroxide charged was e tion material compatible with thereaction system can be prexhhately theoretlcal- Thfi Product was f goodpurity. employed. 7 p

The accompanying drawing is a flow sheet showing examples In and IVschematically one form of reactor and associated equip- In the nextthree examples the residence time mainment for use in practicing theprocess of this invention. tained in each case was about seven minutes.These ex- In the drawing the reactor 10 is in the form of a cross ampleseach involved the continuous feed of aqueous of suitable constructionmaterial, such as aluminum. Leg solutions of reactants at the ratesindicated in the table 11 of the reactor is provided with a thioureasolution feed which follows. Examples II and III were of one hour line12 communicating with a metering pump (not shown) duration. Example IVof 1 /2 hours duration. In all exfor controlling the flow of thethiourea solution stream amples substantially stoichiometric amounts ofthiourea supplied by this line to the reactor 10. Leg 14 of reactor andhydrogen peroxide were introduced into the reactor 10 has leadingthereinto a. hydrogen peroxide feed line which was a cylindrical housingequipped with an agitator 1S communicating with a metering pump (notshown) for into which the reactant streams were introduced at thecontrolling the feed of the hydrogen peroxide to reactor rates indicatedand the reaction mixture continuously 10. Thermocouple 17 is positionedin leg 18 to measure with-drawn therefrom. The hydrogen peroxide usedwas the temperature in the reactor 10. The fourth leg 19, ofconcentration; the amounts added gave areaction which is the dischargeconduit for the reaction mixture m xture containing about 6% hydrogenperoxide.

TABLE Ex. No. Thioureu Flow Vol. Thiourea Mols Thiouroa H2O: Flow Vol.H202 Mols H202 pH of Rcac- Yield of 'IDO Rate, ml./min. Added, 1111.Added Rate, rut/min. Added, ml. Added tion Mixture 40 2, 4-20 4.1 5.7338 5 9 5.0 i 93 40 2, 420 4.1 5.7 338 5 0 5.0 90 40 3,610 6.15 5.7 436s 5 5.5 i 90 communicates with a coil 21, the main body of which isimmersed in the cooling liquid in vessel 22 fragmentarily shown in thedrawing. This vessel can be supplied with cooling water or othersuitable cooling medium; water is indicated for illustrative purposes onthe drawing. Cold water is supplied through line 23 to maintain aconstant level of cooling water in tank 22, the level of which iscontrolled by the overflow tube 24 which leads to the sewer or othersuitable disposal point.

Cooling coil 21 communicates with a heat exchanger 25 where furthercooling of the reaction mixture is effected employing a suitable coolingmedium such, for example, as ice water. Heat exchanger 25 effectscooling of the reaction mixture to a temperature of about 0 C. From heatexchanger 25 the cooled slurry is discharged through exit line 26 into afilter 27. In this filter, product TDO crystals 28 are separated fromthe filtrate which is discharged through line 29. When employing anequilibrium mixture of thiourea and ammonium thiocyanate, the latterremains in solution in the filtrate; it can be recovered by evaporatingthe solution and heated to produce equilibrium mixture for furtherprocessing.

It will be noted that the present invention provides a continuousprocess for producing TDO in simple and compact equipment. The need forthe large equipment required for batch operations on a commercial scaleis eliminated. In the continuous process of this invention undesirableside reactions are reduced to a minimum and the T DO pro-duct obtainedis chemically pure and shows excellent storage stability.

Since certain changes in carrying out the above continuous process,which embodies the invention, can be made without departing from itsscope, it is intended that all matter contained in the above descriptionor shown in the accompanying drawing shall be interpreted asillustrative and not in a limiting sense.

What is claimed is:

1. The continuous process of synthesizing a mino-iminomethane-sulfuricacid, which comprises continuously feeding an aqueous solution ofthiourea containing from 0.7% to 10% by weight of thiourea to a reactionzone, concurrently continuously feeding an aqueous solution of hydrogenperoxide into said reaction zone and producing an intimate mixture ofthese reactants, the amount of hydrogen peroxide thus introduced beingsuch as to produce a reaction mixture containing from 1.5% to 12% byweight of hydrogen peroxide, continuously withdrawing reaction mixturefrom said reaction zone, maintaining the reaction zone at a temperaturewithin the range of from 5 C. to 80 C., controlling the rate of flow ofsaid aqueous solution of thiourea and said stream of hydrogen peroxideand the rate of withdrawal of the reaction mixture from said reactionzone so that the residence time of the reactants and the reactionproduct in said reaction zone is within the range of from 0.1 second tominutes and the pH of the reaction mixture is from 5 to 7, cooling thereaction mixture immediately upon withdrawal from the reaction zone thusforming a slurry of amino-iminomethane-sulfinic acid crystals, andcontinuously separating the aminoiminornethane-sulfinic acid crystalsfrom the slurry.

2. The continuous process as defined in claim 1, in which the aqueoussolution containing thiourea is a solution of an equilibrium mixture ofthiourea and ammonium thiocyanate.

3. The continuous process as defined in claim 1, in

6 which the temperature in the reaction zone is maintained within therange of from 5 C. to 15 C.

4. The continuous process as defined in claim 1, in 'Which thetemperature in the reaction zone is maintained within the range of from5 C. to 15 C., and the reaction mixture is cooled to a temperature ofabout 0 C. immediately after it is removed from the reaction zone.

5. The continuous process as defined in claim 1, in which the thioureasolution stream contains about 10% by weight of thiourea and thehydrogen peroxide stream fed to the reactor contains about by Weight ofhydrogen peroxide.

References Cited UNITED STATES PATENTS 2,150,921 3/1939 Havas 260-5002,347,446 4/ 1944 Walker 260500 2,783,272 2/1957 Young 260500 LEONZITVER, Primary Examiner.

I. E. EVANS, Assistant Examiner.

1. THE CONTINUOUS PROCESS OF SYNTHESIZING AMINO-IMINOMETHANE-SULFINICACID, WHICH COMPRISES CONTINUOUSLY FEEDING AN AQUEOUS SOLUTION OFTHIOUREA CONTAINING FROM 0.7% TO 10% BY WEIGHT OF THIOUREA TO A REACTIONZONE, CONCURRENTLY CONTINUOUSLY FEEDING AN AQUEOUS SOLUTION OF HYDROGENPEROXIDE INTO SAID REACTION ZONE AND PRODUCING AN INTIMATE MIXTURE OFTHESE REACTANTS, THE AMOUNT OF HYDROGEN PEROXIDE THUS INTRODUCED BEINGSUCH AS TO PRODUCE A REACTION MIXTURE CONTAINING FROM 1.5% TO 12% BYWEIGHT OF HYDROGEN PEROXIDE, CONTINUOUSLY WITHDRAWING REACTION MIXTUREFROM SAID REACTION ZONE, MAINTAINING THE REACTION ZONE AT A TEMPERATUREWITHIN THE RANGE OF FROM 5*C. TO 80*C., CONTROLLING THE RATE OF FLOW OFSAID AQUEOUS SOLUTION OF THIOUREA AND SAID STREAM OF HYDROGEN PEROXIDEAND THE RATE OF WITHDRAWAL OF THE REACTION MIXTURE FROM SAID REACTIONZONE SO THAT THE RESIDENCE TIME OF THE REACTANTS AND THE REACTIONPRODUCT IN SAID REACTION ZONE IS WITHIN THE RANGE OFFROM 0.1 SECOND TO10 MINUTES AND THE PH OF THE REACTION MIXTURE IS FROM 5 TO 7, COOLINGTHE REACTION MIXTURE IMMEDIATELY UPON WITHDRAWAL FROM THE REACTION ZONETHUS FORMING A SLURRY OF AMINO-IMINOMETHANE-SULFINIC ACID CRYSTALS, ANDCONTINUOUSLY SEPARATING THE AMINOIMINOMETHANE-SULFINIC ACID CRYSTALSFROM THE SLURRY.